In a number of fields of technology, moving machine elements or webs or strip-shaped elements to be treated are so arranged or displaced that they form a converging gap. The moving surfaces of the element or material entrained a fluid (gas, liquid) into the gap with a volume or pressure giving rise to negative effects in the event certain boundary conditions are exceeded. The geometry of the gap is either shape stable or of variable shape. Stable shape gaps are formed for example between two rotating rollers whose axes are parallel or between a moving web supported by one guide element and a further element. Shape-variable gaps are formed for example by one element (drive roller, guide roll or roller, winding roll, etc.) and a web running to it or also between two webs running together.
An important technical field of use for the process and the device is the production and further processing of web-shaped materials (paper, cardboard, synthetic resin foils) with which it is frequently required to bring the web into very close contact with an element (roller, roll, or fixed guide element, winding roll).
Especially at high web speeds, the air entrained into the converging gap can be problematical. The entrained air creates an overpressure which presses the web away from the element and which is thus the cause of undesired negative effects. Thus in the roll up of webs of material to wound rolls, as the oncoming web passes onto the wound roll air can be wound into the roll in an undesired manner. With drive rollers the web may begin to float because of an entrained air cushion. The adhesion and thus the drive force is lost. With rollers or other guide elements which serve to control web travel, a floating occurs which interferes with the controlled movement and causes the web to shift laterally. With cooling or heating rollers for webs the entrained air volume reduces the heat exchange with the roll surface and the web and also the heating or cooling efficiency. Since heating and cooling roller groups usually also serve as the traction stations for the web, good adhesion is also required to impart tension to the web. In the cooling of material webs for example paper or cardboard, the entrained air cushion is detrimental to the transfer of the coating material from the applicator to the web.
There are various methods known to counteract the negative effects which find their origin in the entrainment of air into a gap between a web and a further element.
Thus in the winding of webs into wound rolls, pressure rollers are used which squeeze the air out as the web meets the wound roll. Pressure rollers are expensive in their construction and can mark sensitive materials. They can also influence the roll hardness during the wind up in an undesired manner. An improvement in contact with pressure rollers is not possible when the web is coated or when folds may form during the pressing. With drive rollers it is known to improve the contact by a corresponding shaping of the roller surface, for example, by the application of grooves. All of these techniques have in common that they are either relatively expensive to construct or can give rise to undesirable side effects.
A further field of use of the invention is the coating of material webs in the process in which a liquid coating material is applied by means of an applicator element, especially an applicator roller which forms a narrowing application gap with the web. The liquid coating material fed into the application gap of the application element builds up a pressure which influences the quantity applied to the web. An excessively high pressure in the gap, in the case of bibulous paper or cardboard webs, causes the coating material, especially pigment containing coloring agents to penetrate in undesirably large quantities into the web.
The invention thus has as its object the provision of a method by which the volume or the pressure of a fluid entrained into a gap by moving surfaces can be positively reduced in a simple manner without giving rise to undesirable side effects. This object is achieved with the features of patent claim 1.
The separating element used in accordance with the invention effects a pressure or volume reduction in the gap without contacting moving surfaces for example of the web or of another element forming the gap. The separating element requires neither a mechanical drive nor a special energy supply. It enables the possibility of influencing by itself through its shape and position the pressure of volume relationships in the gap. Thus, for example, in the winding up of a material web, even with a reduced quantity of entrained air, the desired winding quality can be maintained.
The invention makes use of the physical effect that the transition between two streams separated by a partition into two passages of reduced height to only one stream in one passage of greater height, results in a pressure reduction depending upon the viscosity of the flowing fluid, the length and height of the passages and the flow velocity and/or a reduction in the volume flow of the fluid into the gap.
The dependent claims contain preferred variants and embodiments of the invention in its use in the production and processing of web shaped materials.